Checkout payment redirect problem

Hi there,
I’ve recently had issue with the payment button on my website. The add-listing and preview-listing pages display fine, but the ‘Confirm Preview & Go to Checkout’ button no longer directs users to the Paypal page. Instead it redirects me back to my homepage and nothing happens. (There is no record of pending invoices or draft listings visible from within the WP admin area).

I’ve tried the following to remedy the situation without success:

1. Edit .htaccess file back to it’s standard content i.e. no redirects etc. Result = no change / still redirects back to homepage.
2. Remove any redirect, profile, member plugins, and other major plugins on my WP install. Result = no change / still redirects to homepage.
3. Update my Geodirectory plugins and update settings in the Geo directory settings area. I’ve tried changing Paypal merchant ID and email address etc. and tried Sandbox and live environments. Result = no change /still redirects to homepage.
4. Tried adding a second transaction route e.g. 2Checkout. Result = no change / still redirects to homepage. It also doesn’t come up with the option to select payment type on the ‘preview page’

I’ve looked through the support forums here, but can’t seem to pinpoint the problem. I’ve attempted to check my SSL cert, but wasn’t sure what i could do with this anyway. I’m not sure how to check/debug so I’m out of options on what else to do, could you please help?

I don’t know when it stopped redirecting. I did change both .htaccess and SSL recently, but i don’t know if it wasn’t working before this. Hopefully it’s something obvious & easy to fix.

Thanks for any help you can give. Great plugins BTW.

Thank you.
Steve